The present invention relates to a cutting tool insert coated with a metal oxide multilayer with improved resistance to flaking and to plastic deformation, particularly suitable for machining of steel by turning, milling, drilling or by similar chip forming machining methods, and a method of making the same.
When cemented carbide cutting tool inserts are used in the machining of metals, the insert is worn by different mechanisms such as abrasive and chemical wear, chipping and fracturing of the cutting edge. Some metals are more difficult to machine than others since, in addition to the above mentioned wear mechanisms, also adhesive wear and resulting flaking of coatings, can occur. Examples of such metals are stainless steel and low carbon steel. Adhesive wear results when smearing materials during the cutting operation continuously adhere to and tear off material from the cutting edge, which shortens tool insert life. Furthermore, when cutting at high cutting speeds, the thermal energy transferred to the cutting edge is considerable and the insert edge may partly or entirely deform plastically. This mode of deterioration of the cutting edge is known as plastic deformation wear.
Multilayer coatings comprising first and second layers of different materials which are alternately laminated on the substrate, each of the first coating layers having a first thickness and each of the second coating layers having a second thickness are known in the art. The two layers should preferably have a different crystal structure and/or at least different lattice spacings. One example of such a structure is when an Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in a (Al2O3+TiN)x multilayer structure see e.g. Proceedings of the 12:th European CVD Conference page pr. 8-349. GB 2048960A discloses a multilayer coating with a multiplicity of alternating layers of 0.02 to 0.1 μm consisting of hard material of different compositions.
U.S. Pat. No. 5,827,570 discloses a cemented carbide or ceramic substrate with a wear resistant composite ceramic coating comprising a doped two-phase metal oxide layer comprising a continuous metal oxide phase, comprising aluminum oxide, yttrium oxide or zirconium oxide, and a discontinuous metal oxide phase, comprising a dispersed, discrete second phase of aluminum oxide, yttrium oxide or zirconium oxide.
In U.S. Pat. No. 4,749,629 the integration of ZrO2 is done by using a laminated oxide coating, with a total thickness of 0.3-20 μm, consisting of alternating different oxide layers such as Al2O3, ZrO2 and Y2O3 and where each oxide layer has a thickness within 0.1-3 μm.
Requirements for high surface finish of the machined component only allow inserts with a clean, smooth edge line with very little developed wear to be used. It is becoming more and more difficult for the machine operator by a naked eye to differentiate between a little used and an unused cutting edge (“used edge identification”). This is particularly difficult if the top layer is Al2O3, making the top layer appear dark gray or black. By mistake, using a used insert edge, e.g., during an unmanned night shift run, may cause component rejection or even unwanted production stops. Edge identification can more easily be done if the insert has a top layer of TiCxNyOz or in particular if the top layer is a goldish color TiN-layer.
EP-B-693574 discloses a cutting tool insert having an outermost Al2O3 layer on the rake face and cutting edge line, because of its ability to withstand diffusion type wear. The clearance face has an outermost MeCxNyOz type layer, where Me is a metal selected from the group consisting of metals in the groups IVB, VB, VIB of the periodic table (further on denoted TiCxNyOz), because of its high resistance to flank wear and the possibility for “used edge identification”. This is obtained by mechanically removing the TiCxNyOz layer either from only the edge line or from both the rake face and the edge line.